The invention generally relates to X-ray apparatus, and more particularly relates to size sensing X-ray cassette trays that are used with X-ray collimation systems to adjust the size of the X-ray beam in accordance with the size of the X-ray cassette mounted on the tray.
With X-ray systems, it is common to use X-ray cassettes that mount on X-ray trays or holders. More specifically, the unexposed film or other sensitive material is placed inside a rectangular cassette that is positioned on a cassette tray. In order to center and hold cassettes of various different sizes, trays typically have front and back clamps that slide in unison toward and away from the center of the tray. The loaded cassette tray is then inserted into the X-ray system.
As is well known, it is desirable to adjust or collimate the X-ray beam to the size of the film being used so as to avoid unnecessary radiation. For this purpose, it has been common to use X-ray trays that sense the size of the cassette so that the system can automatically collimate the beam in accordance therewith. Once such system is described in U.S. Pat. No. 4,416,020. The cassette tray has a straight linear potentiometer that runs adjacent and parallel to the slide track of the clamp. The wiper of the potentiometer is mounted to the slide of the clamp such that the electric potential at the output of the potentiometer is directly related to the length of the X-ray cassette that is clamped. Similarly, a side sensing finger moves in a straight slot that is orthogonal to the clamp slide, and a second straight linear potentiometer runs adjacent and parallel to the slot. Accordingly, the second potentiometer provides an electric potential that is directly related to the width of the X-ray cassette. With such arrangement, however, the straight potentiometers must be relatively long so as to be able to sense the dimensions of the smallest and the largest cassette sizes. Further, there may be a tendency for the wipers to bounce because they are mounted to slides, and such tendency increases with age of the tray. Another disadvantage is that the potentiometers are adjacent to the open slots and therefore are subject to environmental conditions that can cause degradation of the electrical contact. Also, the mechanical linkage which drives the sensing finger in a straight slot from a rotating arm may be unreliable and subject to sticking or jamming.
Another prior art approach is described in U.S. Pat. No. 3,764,808. The side sensing finger moves in an arcuate path thereby eliminating a mechanical linkage between a rotating arm and a straight slot sensor. However, recognizing that the setting of a rotary potentiometer is linearly related to the sine of the angle formed by the side of the cassette and a line from the point of contact of the sensing finger through the pivot post, mechanical compromises were made in an attempt to minimize the nonlinearity between the potentiometer setting and the cassette dimensions. For example, the pivot point was selected so as to keep the angle small so as to minimize the deviation from linearity. However, such apparatus only partially compensates for the sine error.